Clarify dateDiff() subtraction direction and result sign in docs

[spmallette]

The dateDiff() documentation described the step as returning "the difference between two Dates" but never stated which operand is subtracted from which, so the sign of the result was unexplained. A newcomer running the reference example sees a negative value with no indication of why.

This change documents the direction and sign in both places that describe the step:

• docs/src/reference/the-traversal.asciidoc (dateDiff-step): adds prose stating the result is computed as the incoming traverser date minus the argument date, where a positive result means the incoming date is later and a negative result means it is earlier, and explains why the example returns -86400000 (the incoming date is one day earlier than the argument). The live example block already renders its ==> -86400000 output.
• docs/src/dev/provider/gremlin-semantics.asciidoc (dateDiff()): updates the Description to state the same direction and sign meaning, and replaces the ambiguous "The first result returned from the traversal will be subtracted with the incoming traverser" wording with an explicit "result = incoming DATETIME - argument" formulation for both the value and dateTraversal arguments.

This is a documentation-only clarification; the behavior is unchanged and was verified against DateDiffStep (Duration.between(argument, incoming)). The docs build renders both sections cleanly.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.47%. Comparing base (a28cd1f) to head (24a3229).
⚠️ Report is 346 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3552      +/-   ##
============================================
+ Coverage     76.35%   76.47%   +0.11%     
- Complexity    13424    14288     +864     
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  Files          1012     1036      +24     
  Lines         60341    64561    +4220     
  Branches       7075     7656     +581     
============================================
+ Hits          46076    49371    +3295     
- Misses        11548    12098     +550     
- Partials       2717     3092     +375     

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